Radionuclide angiocardiography is a noninvasive technique which provides noninvasive assessment of hemodynamics and ventricular function. The purpose of this study is to evaluate the clinical usefulness of radionuclide measurement of left ventricular function during rest and exercise in patients with congenital heart disorders. Serial measurements of ventricular function will be obtained in children with congenital valvular stenosis and insufficiency to evaluate the usefulness of these measurements in timing of operation. Patients who have previously undergone operative correction of tetralogy of Fallot will undergo rest and exercise measurement of right and left ventricular function. The purpose of these observations will be to assess whether the age at the time of operation or technical details of the operative approach can be related to exercise-induced abnormalities of ventricular function. Measurements will be obtained in children with essential hypertension to document the effect of chronic increase in afterload to the left ventricle on cardiac performance during exercise. Measurements of cardiac function will be obtained in children with sickle cell anemia to document the magnitude and frequency of cardiac abnormalities which appear during exercise in these patients. These applications of radionuclide techniques to study cardiovascular function during exercise should greatly enhance the understanding and management of patients with valvular heart disorders, hypertension, sickle cell, cardiomyopathy and specific surgical disorders.